1. Field of the Invention
The present invention relates to a process for the desulfurization of a full boiling range fluid catalytic cracked naphtha. More particularly the present invention employs catalytic distillation steps which reduce sulfur to very low levels, makes more efficient use of hydrogen and causes less olefin hydrogenation for a full boiling range naphtha stream.
2. Related Information
Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling ranges which determines the composition. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
The composition of untreated naphtha as it comes from the crude still, or straight run naphtha, is primarily influenced by the crude source. Naphthas from paraffinic crude sources have more saturated straight chain or cyclic compounds. As a general rule most of the xe2x80x9csweetxe2x80x9d (low sulfur) crudes and naphthas are paraffinic. The naphthenic crudes contain more unsaturates and cyclic and polycylic compounds. The higher sulfur content crudes tend to be naphthenic. Treatment of the different straight run naphthas may be slightly different depending upon their composition due to crude source.
Reformed naphtha or reformate generally requires no further treatment except perhaps distillation or solvent extraction for valuable aromatic product removal. Reformed naphthas have essentially no sulfur contaminants due to the severity of their pretreatment for the process and the process itself.
Cracked naphtha as it comes from the catalytic cracker has a relatively high octane number as a result of the olefinic and aromatic compounds contained therein. In some cases this fraction may contribute as much as half of the gasoline in the refinery pool together with a significant portion of the octane.
Catalytically cracked naphtha (gasoline boiling range material) currently forms a significant part (≈⅓) of the gasoline product pool in the United States and it provides the largest portion of the sulfur. The sulfur impurities may require removal, usually by hydrotreating, in order to comply with product specifications or to ensure compliance with environmental regulations. Some users wish the sulfur of the final product to be below 50 wppm.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical HDS unit:
RSH+H2xe2x86x92RH+H2Sxe2x80x83xe2x80x83(1)
RCl+H2xe2x86x92RH+HClxe2x80x83xe2x80x83(2)
2RN+4H2xe2x86x922RH+2NH3xe2x80x83xe2x80x83(3)
ROOH+2H2xe2x86x92RH+2H2Oxe2x80x83xe2x80x83(4)
Typical operating conditions for the HDS reactions are:
After the hydrotreating is complete the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the now desulfurized naphtha. The loss of olefins by incidental hydrogenation is detrimental by the reduction of the octane rating of the naphtha and the reduction in the pool of olefins for other uses.
In addition to supplying high octane blending components the cracked naphthas are often used as sources of olefins in other processes such as etherifications. The conditions of hydrotreating of the naphtha fraction to remove sulfur will also saturate some of the olefinic compounds in the fraction reducing the octane and causing a loss of source olefins.
Various proposals have been made for removing sulfur while retaining the more desirable olefins. Since the olefins in the cracked naphtha are mainly in the low boiling fraction of these naphthas and the sulfur containing impurities tend to be concentrated in the high boiling fraction the most common solution has been prefractionation prior to hydrotreating. The prefractionation produces a light boiling range naphtha which boils in the range of C5 to about 250xc2x0 F. and a heavy boiling range naphtha which boils in the range of from about 250-475xc2x0 F.
The predominant light or lower boiling sulfur compounds are mercaptans while the heavier or higher boiling compounds are thiophenes and other heterocyclic compounds. The separation by fractionation alone will not remove the mercaptans. However, in the past the mercaptans have been removed by oxidative processes involving caustic washing. A combination oxidative removal of the mercaptans followed by fractionation and hydrotreating of the heavier fraction is disclosed in U.S. Pat. No. 5,320,742. In the oxidative removal of the mercaptans the mercaptans are converted to the corresponding disulfides.
U.S. Pat. No. 5,597,476 discloses a two step process in which naphtha is fed to a first distillation column reactor which acts as a depentanizer or dehexanizer with the lighter material containing most of the olefins and mercaptans being boiled up into a first distillation reaction zone where the mercaptans are reacted with diolefins to form sulfides which are removed in the bottoms along with any higher boiling sulfur compounds. The bottoms are subjected to hydrodesulfurization in a second distillation column reactor where the sulfur compounds are converted to H2S and removed.
In the present invention it has been discovered that during processing if the H2S is not removed from the catalyst zones quickly problems arise. The H2S can recombine to form mercaptans thus increasing the amount of sulfur in the product. Also the presence of H2S can cause more of the olefins to be saturated losing octane and consuming hydrogen.
It is an advantage of the present invention that a full boiling range naphtha stream is hydrodesulfurizated by splitting it into boiling range fractions which are treated to simultaneously hydrodesulfurize and fractionate the fractions. It is a further advantage of the present invention that the sulfur may be removed from the light portion of the stream to a heavier portion of the stream without any substantial loss of olefins. It is a particular feature of the present invention that substantially all of the sulfur contained in the naphtha is ultimately converted to H2S which is quickly removed from the catalyst zones and easily distilled away from the hydrocarbons minimizing production of recombinant mercaptans and with reduced hydrogenation of olefins.
Briefly in the present invention subjects a full boiling range naphtha stream containing organic sulfur compounds and diolefins is fractionated in a first distillation column reactor by boiling a portion of the stream containing lower boiling organic sulfur compounds, generally mercaptans and diolefins into contact with a Group VIII metal hydrogenation catalyst under conditions to form sulfides. A lower boiling portion of the stream, having a reduced amount of organic sulfur compounds and diolefins is recovered as light naphtha overheads. The sulfides formed by the reaction of the mercaptans and diolefins are higher boiling and are removed from the column in a heavier bottoms. The heavier bottoms comprise that portion of the streams not removed as overheads. Although hydrogen is present in this column, it is present in an amount to maintain the catalyst is the hydride form for the sulfide reaction and very little of the olefins present are hydrogenated. Furthermore, the presence of diolefins in this fraction deters olefin hydrogenation, since the diolefins are preferentially hydrogenated.
The heavier bottoms and hydrogen are fed to a second distillation column reactor, where the heavier bottoms are fractionated into an intermediate naphtha portion and a heavy naphtha portion. The organic sulfur compounds in the intermediate naphtha portion are brought into contact with hydrogen in the presence of a hydrodesulfization catalyst under conditions to convert the organic sulfur compounds to H2S which is removed with the intermediate naphtha portions as an intermediate naphtha overheads. Higher boiling organic sulfur compounds originally present in the stream and the sulfides produced in the first column are removed with a heavy naphtha portion as bottoms.
The heavy naphtha and hydrogen are preferably fed to a third distillation column reactor where the entire heavy naphtha portion is contacted with hydrodesulfurization catalyst under conditions to convert the remaining organic sulfur compounds and the sulfides formed in the first distillation column reactor into H2S which is removed as overheads while the heavy naphtha is removed as bottoms.
The advantage of this process is that the separation of the heavies from the first column into two fractions which are separately hydrodesulfurized, results in the intermediate naphtha portion not being subjected to contact with the H2S liberated from the heavy naphtha portion and the H2S is more quickly removed from contact with the catalyst. Quicker removal of the H2S from the reaction zone reduces the opportunity for the recombination to occur.
It can be appreciated that three distillation column reactors produces a substantial improvement in sulfur removal, however if even greater reduction is required, division of the full boiling range naphtha into smaller fractions for hydrodesulfization in more than two columns will produce further reduction of organic sulfur compounds. The use of more than two distillation column reactors for hydrodesulfurization of portions of the heavies bottoms from the first column is contemplated as within the present invention.
As used herein the term xe2x80x9cdistillation column reactorxe2x80x9d means a distillation column which also contains catalyst such that reaction and distillation are going on concurrently in the column. In a preferred embodiment the catalyst is prepared as a distillation structure and serves as both the catalyst and distillation structure.
The sulfur compounds produced in the first distillation column reactor by the reaction of mercaptans and diolefins are organic sulfur compounds, however, for the purposes of describing and claiming the present invention the organic sulfur compounds, other than mercaptans, contained in the full boiling range naphtha stream feed to the present process are designated as xe2x80x9corganic sulfur compoundsxe2x80x9d and the sulfur compounds produced by the reaction mercaptans and diolefins are designated as xe2x80x9csulfidesxe2x80x9d. The term xe2x80x9csulfur compoundsxe2x80x9d is used herein to generically include the mercaptans, sulfides and organic sulfur compounds.